Certain metallic alloy compositions are known for their magnetic properties. Various applications exist for the use of such alloys within industry. The rapidly expanding use of such alloys has also extended into such markets as electronic article surveillance (EAS) systems. Many of these newer markets require alloys with superior magnetic properties at reduced costs such that the items within which they are employed can be discarded subsequent to their use.
EAS systems can be operated with markers as described in U.S. Pat. Nos. 4,510,489, 5,313,192, and 5,351,033, among others. These markers generally contain, as the operative control means within the marker itself, a semi-hard magnetic element and a soft magnetic element. The semi-hard magnetic element as described by the present invention is a component having a coercivity in the range of about 10-200 oersteds and a remanence, determined after the element is subjected to a DC magnetization field that magnetizes the element substantially to saturation, of about 7-13 kilogauss.
In the tag of the U.S. Pat. No. 4,510,489, a semi-hard magnetic element is placed adjacent to a magnetostrictive amorphous element. By magnetizing the semi-hard magnetic element substantially to saturation, the resultant remanence magnetic induction of the magnetic element arms or activates the magnetostrictive element so that it can mechanically resonate or vibrate at a predetermined frequency in response to an interrogating magnetic field.
The mechanical vibration results in the magnetostrictive element generating a magnetic field at the predetermined frequency. The generated field can then be sensed to detect the presence of the tag. By demagnetizing the semi-hard magnetic element, the magnetostrictive element is disarmed or deactivated so that it can no longer mechanically resonate in response to the applied field.
The metallic alloy compositions that constitute permanent magnets are characterized by various performance properties such as coercive level, H.sub.c, and residual induction, B.sub.r. The coercive level is a measure of the resistance of the magnet to demagnetization and the residual induction is a measure of the level of induction possessed by a magnet after saturation and removal of the magnetic field. Superior magnetic properties can be obtained by using a ferrous alloy containing chromium and cobalt. However, the presence of cobalt typically makes such alloys prohibitively expensive and thus impractical in various end uses, such as elements in markers used in EAS systems.
Certain of the newer magnetic markets further require the preparation of the alloy into a relatively thin strip of material such that the magnetic properties are provided in an economical fashion. As the demand for increasingly thin magnetic strips increases, the selection of metallic alloys possessing the required magnetic properties while also possessing the necessary machinability and workability characteristics to provide the desired shapes, becomes exceedingly difficult. For example, ferrous alloys having carbon contents of about 1 weight percent and chromium contents of about 3-5 weight percent have been shown to exhibit advantageous magnetic properties. However these alloys are mechanically hard and cannot be rolled easily to the required thickness due to either initial hardness or high levels of work hardening during processing.
A need therefore exists in the permanent magnet art, and particularly in the EAS systems art, for thin magnetic strips having superior magnetic properties without the need for cobalt and other expensive components in the alloy compositions constituting the magnetic strip. The magnetic strips should be made from alloy compositions which are amenable to processing of the alloy into the thin strips required by many industrial uses, especially those below about 0.005 inches in thickness.